Current methods for producing focused or directed ultrasonic stress waves in a sample material involve, in most part, the use of piezoelectric devices. Such devices are used individually, with lenses or in arrays the latter being capable of beam direction by varying the phases of different array elements. The piezoelectric devices provide beam patterns which are difficult to predict and to reproduce.
The array technique creates side lobes which direct a certain portion of the beam in a direction other than that desired. Both the individual piezoelectric devices and array systems require direct physical contact with the sample to couple ultrasonic waves therein. The nature and strength of the contact can have large and unpredictable effects on the waves which are transmitted.